1. Field of the Invention
The present invention relates to improvements in methods and apparatus for producing photographic-type transparencies of high density. It also relates to the color transparencies produced by such methods and apparatus.
2. Background Art
In U.S. Pat. No. 4,345,012 issued to Hirsch et for producing high density transparencies of a single color. Such a process involves the steps of forming charge images of identical shape but opposite polarity on opposite sides of a transparent dielectric film, and developing such images with pigmented electroscopic particles of opposite polarity. When viewed through the film, the density of the two developed images add together to provide a denser visible image than might be achieved were the pigment applied to only one side of the film.
According to the disclosure of the above patent, the two charge images of opposite polarity are produced by arranging the transparent film in close proximity to a photoconductive layer having a grounded backing electrode. While an electric field is established across the film, the photoconductive layer is imagewise exposed through the film. While this image-forming process is theoretically possible to perform and is advantageous from the standpoint that the opposing images are in perfect registration, it may be appreciated that this process is difficult to control and to repeat with good results. Moreover, to develop the charge images on the film, access is required to both sides of the film. In the disclosed embodiment, development of the charge images is effected by immersing the entire film in a liquid developer having toner particles of opposite polarity. Over and above these disadvantages, the process disclosed does not lend itself well to automation or to the production of multicolor transparencies.